What if paper currency frequently changes appearance to prevent time travelers from going back in time and becoming rich?
It could easily be countered by opening up a bank account in the past and letting the money accumulate wealth via interest in teh future.
Except there is this thing that some states have passed. If an account has been untouched for way too long (Mostly by dead people) then the local government takes it. This is a overly simplified explanation of the process, but you get the picture.
Then make a small withdrawl from the account right before it becomes government property.